Blowout preventers are used to control pressure within oil and gas wells during drilling and completion of the well. Annular blowout preventers, the type to which the present invention relates, include a single annular resilient packing positioned within a body having means to cause the packing to be moved into sealing engagement with a portion of a pipe string extending through the preventer or against itself to seal a well when no pipe string extends through the preventer.
An annular blowout preventer is shown and described in U.S. Pat. No. 3,572,627 to Jones et al. The packer has a resilient annulus with a circular series of rigid inserts extending through the packer and having irising end flanges on both ends which move to provide end support for the resilient annulus when it is compressed radially inward by pistons. These pistons are positioned around the resilient annulus to provide a direct inwardly directed radial force on the resilient annulus. Other annular preventers have utilized an axial compression of the resilient annulus to cause it to move radially inward to sealing positions. U.S. Pat. Nos. 4,099,699, 2,287,205, 3,667,721, 3,323,773, 2,148,844, 2,812,197 and 2,846,178 are examples of annular blowout preventers of the prior art.
The application of an axial force on the resilient annulus of an annular blowout preventer is simpler and less expensive than the application of a relatively uniform radial force to the resilient annulus. The use of an axial force with the resilient annulus of the above cited Jones et al patent would not be suitable as the rigid inserts would prevent effective compression of the annulus.